Energy Transfer in Supramolecular Heteronuclear Lanthanide Dimers and Application to Fluoride Sensing in Water

In the presence of fluoride anions, [LnL(H₂ O)]⁺ complexes, based on the coordination of a lanthanide (Ln) cation into the cavity of a C_2v symmetrical cyclen-based ligand (L), self-assemble in water to form [(LnL)₂ F]⁺ dimers. The crystal structures of the Yb hydrated monomer and of the fluorinated dimer are reported and analyzed to unravel the impact of the cumulative effect of weak hydrogen bonding and aromatic stacking interactions in the supramolecular assembly. The assembly is stable over a broad range of pH 3-8. A combination of equimolar amounts of Eu and Tb complexes led to a quasistatistical mixture of homo- and heterodimers, as observed by using electrospray mass spectrometry. In the heterodimers, selective excitation into the ⁷ F₆ →⁵ D₄ absorption band of the Tb center at λ=488 nm allowed the observation of a Tb-to-Eu downshifting energy transfer, not observed in the absence of fluoride ions. Analysis of the excited-state lifetimes of the dimers within the frame of the Förster theory of energy transfer showed the transfer to have an efficiency of 34 %, with a corresponding Förster radius of 4.1 Å; thereby, unraveling the short Ln-Ln distance as a crucial parameter of the energy-transfer process. By using equimolar mixtures of the Tb and Eu complexes, the energy-transfer phenomenon was used for a ratiometric sensing of fluoride anions in water with a detection limit of 17.7 nm.